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217 lines
4.8 KiB
Perl
217 lines
4.8 KiB
Perl
# Backwards.pm
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# Copyright (C) 1999 Uri Guttman. All rights reserved.
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# mail bugs, comments and feedback to uri@sysarch.com
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use strict ;
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package Backwards ;
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use Symbol ;
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use Fcntl ;
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use Carp ;
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use integer ;
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#my $max_read_size = 3 ;
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my $max_read_size = 1 << 13 ;
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# support tied handles. the tied calls map directly to the object methods
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*TIEHANDLE = \&new ;
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*READLINE = \&readline ;
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# create a new Backwards object
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sub new {
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my( $class, $filename ) = @_ ;
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my( $handle, $seek_pos, $read_size, $self ) ;
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# get a new handle symbol and the real file handle
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$handle = gensym() ;
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# open the file for reading
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unless( open( $handle, $filename ) ) {
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carp "Can't open $filename $!" ;
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return ;
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}
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# seek to the end of the file
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$seek_pos = sysseek( $handle, 0, 2 ) ;
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# get the size of the first block to read,
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# either a trailing partial one (the % size) or full sized one (max read size)
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$read_size = $seek_pos % $max_read_size || $max_read_size ;
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# create the hash for the object, bless and return it
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$self = {
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'file_name' => $filename,
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'handle' => $handle,
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'read_size' => $read_size,
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'seek_pos' => $seek_pos,
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'lines' => [],
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} ;
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return( bless( $self, $class ) ) ;
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}
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sub readline {
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my( $self, $line_ref ) = @_ ;
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my( $handle, $lines_ref, $seek_pos, $read_cnt, $read_buf,
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$file_size, $read_size, $text ) ;
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# get the buffer of lines
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$lines_ref = $self->{'lines'} ;
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while( 1 ) {
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# see if there is more than 1 line in the buffer
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if ( @{$lines_ref} > 1 ) {
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# we have a complete line so return it
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return( pop @{$lines_ref} ) ;
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}
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# we don't have a complete, so have to read blocks until we do
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$seek_pos = $self->{'seek_pos'} ;
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# see if we are at the beginning of the file
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if ( $seek_pos == 0 ) {
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# the last read never made more lines, so return the last line in the buffer
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# if no lines left then undef will be returned
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return( pop @{$lines_ref} ) ;
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}
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#print "c size $read_size\n" ;
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# we have to read more text so get the handle and the current read size
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$handle = $self->{'handle'} ;
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$read_size = $self->{'read_size'} ;
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# after the first read, always read the maximum size
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$self->{'read_size'} = $max_read_size ;
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# seek to the beginning of this block and save the new seek position
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$seek_pos -= $read_size ;
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$self->{'seek_pos'} = $seek_pos ;
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sysseek( $handle, $seek_pos, 0 ) ;
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#print "seek $seek_pos\n" ;
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# read in the next (previous) block of text
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sysseek($handle, 0, 1);
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$read_cnt = sysread( $handle, $read_buf, $read_size ) ;
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#print "Read <$read_buf>\n" ;
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# if ( $read_cnt != $read_size ) {
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# print "bad read cnt $read_cnt != size $read_size\n" ;
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# return( undef ) ;
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# }
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# prepend the read buffer to the leftover (possibly partial) line
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$text = $read_buf . ( pop @{$lines_ref} || '' ) ;
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# split the buffer into a list of lines
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# this may want to be $/ but reading files backwards assumes plain text and
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# newline separators
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@{$lines_ref} = $text =~ m[(^.*\n|^.+)]mg ;
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#print "Lines \n=>", join( "<=\n=>", @{$lines_ref} ), "<=\n" ;
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}
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}
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__END__
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=head1 NAME
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Backwards.pm -- Read a file backwards by lines.
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=head1 SYNOPSIS
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use Backwards ;
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# Object interface
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$bw = Backwards->new( 'log_file' ) ;
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while( $log_line = $bw->readline ) {
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print $log_line ;
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}
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# Tied Handle Interface
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tie *BW, 'log_file' ;
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while( <BW> ) {
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print ;
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}
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=head1 DESCRIPTION
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This module reads a file backwards line by line. It is simple to use,
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memory efficient and fast. It supports both an object and a tied handle
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interface.
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It is intended for processing log and other similar text files which
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typically have new entries appended. It uses newline as the separator
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and not $/ since it is only meant to be used for text.
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It works by reading large (8kb) blocks of text from the end of the file,
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splits them on newlines and stores the other lines until the buffer runs
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out. Then it seeks to the previous block and splits it. When it reaches
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the beginning of the file, it stops reading more blocks. All boundary
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conditions are handled correctly. If there is a trailing partial line
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(no newline) it will be the first line returned. Lines larger than the
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read buffer size are ok.
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=head2 Object Interface
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There are only 2 methods in Backwards' object interface, new and
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readline.
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=head2 new
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New takes just a filename for an argument and it either returns the
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object on a successful open on that file or undef.
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=head2 readline
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Readline takes no arguments and it returns the previous line in the file
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or undef when there are no more lines in the file.
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=head2 Tied Handle Interface
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The only tied handle calls supported are TIEHANDLE and READLINE and they
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are typeglobbed to new and readline respectively. All other tied handle
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operations will generate an unknown method error. Do not seek, write or
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do any other operation other than <> on the handle.
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